Fix warning when using _eq_string
Implement string assert as a function

This assertions prints better diagnostics if the comparison fails.
Update documentation
Simplify header
Simplify writing assertions

by introducing a couple helper functions that abstract away some
complexity.
Update documentation
Strip escape sequences if writing to file

This is a rather hacky solution and it would be nice not write them in
the first place.
Properly recognize escape sequences

While there may be escape sequences that will not be recognized and
therefore stripped, libutest uses no such sequence and won't encouter
problems.